This invention relates to a method for treating slag arising from the refining of stainless steel. In particular, the invention proposes a technique for effectively preventing harmful substances from becoming eluted from the slag, which substances cause anxiety in recycling the slag.
Effective utilization of slag which is a by-product formed during steel refining is considered to be a problem of importance by steel manufacturers from the viewpoints of reducing the cost in steel production and also saving the area in a yard provided exclusively for the slag on the plant premise. In order for the slag induced from steel production to be effectively utilized, it is generally predominantly used by recycling into a sintering plant, or as a roadbed material, a reclamation material for civil works and the like. For instance, with regard to the slag applied to the reclamation material for civil works, a process for modifying and solidifying the slag by use of an anti-powdering agent is proposed as described in Kawatetsu Technical Bulletin (1986, vol. 18, No. 1, p. 20-24).
A noticeable difference between the slag formed by refining stainless steel (hereinafter referred to simply as xe2x80x9cstainless steel slagxe2x80x9d) and the slag formed by producing any other steel is that the former slag has chromium oxide in a content of several percentages as determined in terms of Cr. Although this content is not beyond the values regulated for the reclamation of industrial waste, the chromium oxide sometimes elutes as a hexavalent chromium ion (Cr6+) into water. To utilize the stainless steel slag as a roadbed material or a reclamation material for civil works, therefore, it is desired that strict measures be taken such that environmental pollution by no means arises from the elution of Cr6+ even in the distant future.
Several techniques have thus far been proposed which are designed to prevent Cr6+ from being eluted from the stainless steel slag. For example, Japanese Unexamined Patent Application Publication No. 06-171993 discloses a process in which chromium oxide-containing slag is modified by adding an aluminum ash in an amount of 1-30% by weight and a magnesia type material in an amount of 0.5-15% by weight, respectively, based on the total cumulative weight of the stainless slag in a molten state. However, such a process cannot be said to be satisfactory for slag treatment because Cr6+ elutes in some instances. The process further forms dicalcium silicate in a rather large quantity, and in this case, the slag often becomes powdered because the silicate compound invites crystalline transformation.
Furthermore, Japanese Unexamined Patent Application Publication No. 08-104553 discloses a process in which an inert gas is blown into chromium oxide-containing slag in a molten state formed when stainless steel is refined by decarburization and thereafter subjected to reduction treatment, followed by addition of a divalent sulfur-containing material with stirring, such that the concentration of sulfur in the slag is set to be not less than 0.20% by weight.
Even with this technique employed, however, there is still a danger of Cr6+ becoming eluted depending on the treating conditions of the slag. In instances where stainless steel slag is treated, particularly when it contains chromium at as high a concentration as 2.5% by weight or above, the conventional art appears to make it difficult for the elution of Cr6+ to be prevented nearly completely. This fact poses a further problem in that if a region with a high concentration of chromium is locally present in the slag, it is impossible to modify the slag stably.
An object of the present invention is to modify chromium oxide-containing slag formed at a step of refining stainless steel, thereby preventing Cr6+ from elution from the slag in a substantially complete manner. In other words, this object is to propose a method for treating stainless steel slag, which permits the slag to be effectively utilized as a roadbed material or a reclamation material for civil works without suffering any fear of environmental pollution. Above all, a method for the treatment of stainless steel slag is proposed in which even when the concentration of chromium in the slag is high and beyond 2.5% by weight, Cr6+ is substantially completely prevented from being eluted from the slag.
In order to produce stainless steel, a variety of processes have been practically applied which include a process of Electric Furnace-AOD (Argon Oxygen Decarburization), a process of Converter-VOD (Vacuum Oxygen Decarburization), a process of Chromium Ore Melting Reduction-Converter Decarburization) and the like. Using different production conditions, the present inventors have made many inspections as to how Cr6+ elutes into water in various sorts of stainless steel slag resulting from these processes.
Consequently, these inventors have found that when molten slag having a low basicity is mixed with a strongly reductive material containing sulfur or a sulfide so as to increase the concentration of sulfur in the slag, Cr6+ can be almost completely prevented from being eluted. It has also been discovered that this beneficial effect is influenced by the speed of cooling at which the molten slag is cooled after being mixed with the sulfur-containing material and that surprising results can be obtained especially by quenching the slag. Here, the basicity denotes the ratio of the content (% by weight) of CaO in the slag to the content (% by weight) of SiO2 in the slag, i.e., CaO (% by weight)/SiO2 (% by weight) (hereinafter defined as such).
Based on the above-described inventive concepts, the present invention is constituted as follows:
Namely, this invention provides a method for the treatment of slag resulting from the refining of stainless steel, which comprises adjusting slag in a molten state formed at a step of refining stainless steel such that the slag has a basicity in the range of 1.3-4.0, and mixing the slag with a sulfur-containing material wherein the sulfur is zero or minus in valence, thereby setting the concentration of sulfur in the slag to be not less than 0.2% by weight.
As the processes for mixing the slag with the sulfur-containing material in this treatment method, the following three embodiments are preferred; that is, (1) a mixing process in which molten slag having been adjusted in its basicity is allowed to flush in a slag receiver after or during incorporation of the sulfur-containing material in the slag receiver, (2) a mixing process in which molten slag having its basicity adjusted is maintained in a slag receiver capable of heating the slag, followed by addition of the sulfur-containing material to the slag receiver, and (3) a mixing process in which after molten stainless steel is tapped from a refining furnace, the sulfur-containing material is added to molten slag that has been left in the furnace and adjusted in its basicity.
In the treating method according to the present invention, yet another mixing process is preferred in which stainless steel is refined in an electric furnace, and the sulfur-containing material is added with stirring to molten slag having been adjusted in its basicity with molten stainless steel made present in the refining furnace.
In addition, in all of the above mixing processes, the molten slag should preferably be stirred by at least one of a process for rocking a slag-accommodated receiver, and a process for blowing a gas into the slag.
Besides and preferably, in all of the above mixing processes, FeS should be used as the sulfur-containing material.
Also in all of the above mixing processes, a boron oxide-containing material should preferably be further added at a step of mixing the sulfur-containing material. In this case, the concentration of B2O3 in the slag should more preferably be set to be not less than 0.1% by weight.
Lastly, in all of the above mixing processes, there is provided a process for quenching the slag at such a speed of cooling that the temperature of the slag is decreased within 48 hours from 1200xc2x0 C. to 400xc2x0 C. when the slag is cooled after being mixed with the sulfur-containing material. This quenching process is by far more conspicuously conducive to the desired effects of the present application, and therefore, is especially desirable and important. The quenching noted here should preferably be performed using at least one selected from the group consisting of a cooling process by water grinding, a cooling process by air grinding, and a cooling process by sprinkling of water in an amount larger than 0.05 t/h based on 1 t of the slag flushed in a thickness within 2 m and having a temperature higher than 1200xc2x0 C.